easy. the amount of stress suffered by a piston depends on the
amount of force acting on it (equivalent to the power of the
combustion). now, a larger cc engine has a larger piston, a smaller
cc engine has a smaller piston. if a small cc engine produces more
power than the bigger engine, it's piston will suffer more stress
due to the smaller surface in contact with the combusting
wavefront. likewise, if the bigger engine produces 1000 hps and the
smaller engine produces 750 hps, how to justify how much stress
each piston undergoes?
Pardon me, but it's my understanding that:
Horsepower in bhp = Torque in ft-lbs * RPM / 5252
So... how can there be "no relation to the speed of an engine or
its RPM at that speed"? It's a directly proportional relationship.
It's the reason why sports bikes can have such high BHP, much more
than a normal 1 liter car... due to the high revs! Same thing with
Formula 1 cars. 800+ BHP out of 3 liters normally aspirated engine,
let's see a street car do that!
a nissan march is 1.0l too. are u saying that if its engine can
rev to "sky high" limits like the sports bike, it will be able to
produce as much power? of course not. the amount of power generated
at whichever rpm does not depend on the rpm, but factors such as
compression, cam timings, bore vs stroke design etc.
and this formula?
Horsepower in bhp = Torque in ft-lbs * RPM / 5252
i can tell you that this is a formula relating Torque and
Horsepower. nothing more. Torque, Horsepower and rpms are all
variables. how can u justify that they are all related by the mere
fact that they exist in the same equation? you show me a formula
like this;
Horsepower = (a constant) x rpm
and maybe u have something smart to prove. 
More or less agree with the first statement but not the
remainder, because you're contradicting what you typed earlier!
"the only inter-relatory factor is that on a higher hp engine is
that the engine will have enough power to hit the same speed of a
smaller hp engine within a shorter time"... this statement is
incorrect because you already mentioned that gearing is important
as a "torque multiplier".
i'll go slow with u.
"gearing" is gearing. "horsepower" is horsepower. gearing depends
on the ratios in each perculiar gearbox and horsepower depends on
the efficiency of the engine. how can gearing depend on horepower
or horsepower relate to gearing? u seem like u dunno ur gearboxes
from ur engines. 
using my example, a higher hp engine vs a smaller hps engine
running on the same gearing, of course the bigger hp one win lah
correct anot? (isn't this common sense? why argue with me on this?
)
In other words, if a Ferrari is in 6th gear at a traffic
junction and racing a WRX in 1st gear to the next traffic light, my
money is on the WRX. Horsepower comes into play at higher speeds
where it's needed to overcome aerodynamic drag
of course. no arguments on that. next time remember, never argue with a physics graduate
on equations.
